Download the theory of everything free

Watch The Theory of Everything Free Watch The Theory of Everything online for free - Bilbo and Company are forced to engage in a war against an array of combatants and Download The Theory of Everything Movie Created by. File Size. Share to your Steam activity feed. You need to sign in or create an account to do that.

Sign In Create an Account Cancel. All rights reserved. One then asks how things change if one adds more stars roughly uniformly distributed outside this region. We can add as many stars as we like, but they will still always collapse in on themselves.

We now know it is impossible to have an infinite static model of the universe in which gravity is always attractive. It is an interesting reflection on the general climate of thought before the twentieth century that no one had suggested that the universe was expanding or contracting.

It was generally accepted that either the universe had existed forever in an unchanging state or that it had been created at a finite time in the past, more or less as we observe it today.

Instead, they attempted to modify the theory by making the gravitational force repulsive at very large distances. This did not significantly affect their predic- tions of the motions of the planets. However, we now believe such an equilibrium would be unstable.

If the stars in some region got only slightly near each other, the attractive forces between them would become stronger and would dominate over the repulsive forces. This would mean that the stars would continue to fall toward each other. On the other hand, if the stars got a bit farther away from each other, the repul- sive forces would dominate and drive them farther apart. Another objection to an infinite static universe is normally ascribed to the German philosopher Heinrich Olbers.

In fact, various contemporaries of Newton had raised the problem, and the Olbers article of was not even the first to contain plausible arguments on this subject. It was, however, the first to be widely noted. The difficulty is that in an infinite static universe nearly every line or side would end on the surface of a star. Thus one would expect that the whole sky would be as bright as the sun, even at night. However, if that happened, the intervening matter would eventually heat up until it glowed as brightly as the stars.

In that case, the absorbing matter might not have heated up yet, or the light from distant stars might not yet have reached us. And that brings us to the question of what could have caused the stars to have turned on in the first place. One argument for such a beginning was the feeling that it was necessary to have a first cause to explain the existence of the universe.

Another argument was put forward by St. Augustine in his book, The City of God. He pointed out that civilization is progressing, and we remember who performed this deed or developed that technique. Thus man, and so also per- haps the universe, could not have been around all that long. For otherwise we would have already progressed more than we have.

Augustine accepted a date of about B. It is interesting that this is not so far from the end of the last Ice Age, about 10, B. Aristotle and most of the other Greek philosophers, on the other hand, did not like the idea of a creation because it made too much of divine intervention.

They believed, therefore, that the human race and the world around it had existed, and would exist, forever. They had already considered the argument about progress, described earlier, and answered it by saying that there had been periodic floods or other disasters that repeatedly set the human race right back to the beginning of civilization.

When most people believed in an essentially static and unchanging universe, the question of whether or not it had a beginning was really one of meta- physics or theology. One could account for what was observed either way. Either the universe had existed forever, or it was set in motion at some finite time in such a manner as to look as though it had existed forever. But in , Edwin Hubble made the landmark observation that wherever you look, distant stars are moving rapidly away from us.

In other words, the universe is expand- ing. This means that at earlier times objects would have been closer together. In fact, it seemed that there was a time about ten or twenty thousand million years ago when they were all at exactly the same place. Hubble's observations suggested that there was a time called the big bang when the universe was infinitesimally small and, therefore, infinitely dense.

If there were events earlier than this time, then they could not affect what happens at the present time. Their existence can be ignored because it would have no observational consequences. One may say that time had a beginning at the big bang, in the sense that ear- lier times simply could not be defined.

It should be emphasized that this begin- ning in time is very different from those that had been considered previously. In an unchanging universe, a beginning in time is something that has to be imposed by some being outside the universe. There is no physical necessity for a beginning. One can imagine that God created the universe at literally any time in the past.

On the other hand, if the universe is expanding, there may be physical reasons why there had to be a beginning. One could still believe that God created the universe at the instant of the big bang.

He could even have created it at a later time in just such a way as to make it look as though there had been a big bang. But it would be meaningless to suppose that it was created before the big bang. An expanding universe does not preclude a cre- ator, but it does place limits on when He might have carried out his job. For a long time it was thought that this was the whole universe. It was only in that the American astronomer Edwin Hubble demonstrated that ours was not the only galaxy.

There were, in fact, many others, with vast tracks of empty space between them. In order to prove this he needed to determine the distances to these other galaxies.

We can determine the distance of nearby stars by observing how they change position as the Earth goes around the sun. But other galaxies are so far away that, unlike nearby stars, they really do appear fixed.

Hubble was forced, therefore, to use indirect methods to measure the distances. Now the apparent brightness of a star depends on two factors—luminosity and how far it is from us. For nearby stars we can measure both their apparent brightness and their distance, so we can work out their luminosity.

Conversely, if we knew the luminosity of stars in other galaxies, we could work out their distance by measuring their apparent brightness. Hubble argued that there were certain types of stars that always had the same luminosity when they were near enough for us to measure. If, therefore, we found such stars in another galaxy, we could assume that they had the same luminosity. Thus, we could calculate the distance to that galaxy. If we could do this for a number of stars in the same galaxy, and our calculations always gave the same distance, we could be fairly confident of our estimate.

In this way, Edwin Hubble worked out the distances to nine different galaxies. We live in a galaxy that is about one hundred thousand light-years across and is slowly rotating; the stars in its spiral arms orbit around its center about once every hundred million years.

Our sun is just an ordinary, average-sized, yellow star, near the outer edge of one of the spiral arms. We have certainly come a long way since Aristotle and Ptolemy, when we thought that the Earth was the center of the universe. Stars are so far away that they appear to us to be just pinpoints of light. We cannot determine their size or shape. So how can we tell different types of stars apart? For the vast majority of stars, there is only one correct characteristic feature that we can observe—the color of their light.

Newton discovered that if light from the sun passes through a prism, it breaks up into its component colors—its spectrum—like in a rainbow. By focusing a telescope on an individual star or galaxy, one can similarly observe the spectrum of the light from that star or galaxy. Different stars have different spectra, but the relative brightness of the different colors is always exactly what one would expect to find in the light emitted by an object that is glowing red hot.

This means that we can tell a star's temperature from the spectrum of its light. In the s, when astronomers began to look at the spectra of stars in other galaxies, they found something most peculiar: There were the same character- istic sets of missing colors as for stars in our own galaxy, but they were all shifted by the same relative amount toward the red end of the spectrum.

The only reasonable explanation of this was that the galaxies were moving away from us, and the frequency of the light waves from them was being reduced, or red-shifted, by the Doppler effect. Listen to a car passing on the road. As the car is approaching, its engine sounds at a higher pitch, corresponding to a higher frequency of sound waves; and when it passes and goes away, it sounds at a lower pitch.

The behavior of light or radial waves is similar. Indeed, the police made use of the Doppler effect to measure the speed of cars by measur- ing the frequency of pulses of radio waves reflected off them. In the years following his proof of the existence of other galaxies, Hubble spent his time cataloging their distances and observing their spectra.

At that time most people expected the galaxies to be moving around quite randomly, and so expected to find as many spectra which were blue-shifted as ones which were red—shifted. Every single one was moving away from us. More surpris- ing still was the result which Hubble published in Even the size of the galaxy's red shift was not random, but was directly proportional to the galaxy's distance from us.

Or, in other words, the farther a galaxy was, the faster it was moving away. And that meant that the universe could not be static, as every- one previously thought, but was in fact expanding. The distance between the different galaxies was growing all the time. The discovery that the universe was expanding was one of the great intellec- tual revolutions of the twentieth century. With hindsight, it is easy to wonder why no one had thought of it before. Newton and others should have realized that a static universe would soon start to contract under the influence of gravity.

But suppose that, instead of being static, the universe was expanding. If it was expanding fairly slowly, the force of gravity would cause it eventually to stop expanding and then to start contracting. However, if it was expanding at more than a certain critical rate, gravity would never be strong enough to stop it, and the universe would continue to expand forever. This is a bit like what happens when one fires a rocket upward from the surface of the Earth.

If it has a fairly low speed, gravity will eventually stop the rocket and it will start falling back. On the other hand, if the rocket has more than a certain critical speed—about seven miles a second—gravity will not be strong enough to pull it back, so it will keep going away from the Earth forever.

Yet so strong was the belief in a static universe that it per- sisted into the early twentieth century. Even when Einstein formulated the general theory of relativity in , he was sure that the universe had to be static.

He therefore modified his theory to make this possible, introducing a so- called cosmological constant into his equations. His cosmological constant gave space-time an inbuilt tendency to expand, and this could be made to exactly balance the attraction of all the matter in the universe so that a static universe would result. Only one man, it seems, was willing to take general relativity at face value. On the basis of general relativity and these two assumptions, Friedmann showed that we should not expect the universe to be static.

In fact, in , several years before Edwin Hubble's discovery, Friedmann predicted exactly what Hubble found. The assumption that the universe looks the same in every direction is clearly not true in reality.

For example, the other stars in our galaxy form a distinct band of light across the night sky called the Milky Way. But if we look at distant galax- ies, there seems to be more or less the same number of them in each direction. So the universe does seem to be roughly the same in every direction, provided one views it on a large scale compared to the distance between galaxies.

In , two American physicists, Arno Penzias and Robert Wilson, were working at the Bell Labs in New Jersey on the design of a very sensitive microwave detector for communicating with orbiting satellites.

First they looked for bird droppings on their detector and checked for other possible malfunctions, but soon ruled these out. They knew that any noise from within the atmosphere would be stronger when the detector is not pointing straight up than when it is, because the atmosphere appears thicker when looking at an angle to the vertical.

The extra noise was the same whichever direction the detector pointed, so it must have come from outside the atmosphere. It was also the same day and night throughout the year, even though the Earth was rotating on its axis and orbiting around the sun.

This showed that the radiation must come from beyond the solar system, and even from beyond the galaxy, as otherwise it would vary as the Earth pointed the detector in different directions. In fact, we know that the radiation must have traveled to us across most of the observable universe. Since it appears to be the same in different direc- tions, the universe must also be the same in every direction, at least on a large scale.

We now know that whichever direction we look in, this noise never varies by more than one part in ten thousand. They were working on a suggestion made by George Gamow, once a student of Alexander Friedmann, that the early universe should have been very hot and dense, glowing white hot.

Dicke and Peebles argued that we should still be able to see this glowing, because light from very distant parts of the early universe would only just be reaching us now.

However, the expansion of the universe meant that this light should be so greatly red-shift- ed that it would appear to us now as microwave radiation. Dicke and Peebles were looking for this radiation when Penzias and Wilson heard about their work and realized that they had already found it. Now at first sight, all this evidence that the universe looks the same whichev- er direction we look in might seem to suggest there is something special about our place in the universe.

In particular, it might seem that if we observe all other galaxies to be moving away from us, then we must be at the center of the universe. There is, however, an alternative explanation: The universe might also look the same in every direction as seen from any other galaxy. We believe it only on grounds of modesty. It would be most remarkable if the universe looked the same in every direction around us, but not around other points in the universe.

The situation is rather like steadily blowing up a balloon which has a number of spots painted on it. As the balloon expands, the dis- tance between any two spots increases, but there is no spot that can be said to be the center of the expansion. Moreover, the farther apart the spots are, the faster they will be moving apart.

So it predicted that the red shift of a galaxy should be directly proportional to its distance from us, exactly as Hubble found. The galaxies then start to move toward each other and the universe contracts.

The distance between two neighboring galaxies starts at zero, increases to a maximum, and then decreases back down to zero again. In the second kind of solution, the universe is expanding so rapidly that the gravitational attraction can never stop it, though it does slow it down a bit. The separation between neighboring galaxies in this model starts at zero, and eventually the galaxies are moving apart at a steady speed. Finally, there is a third kind of solution, in which the universe is expanding only just fast enough to avoid recollapse.

In this case the separation also starts at zero, and increases forever. However, the speed at which the galaxies are moving apart gets smaller and smaller, although it never quite reaches zero.

A remarkable feature of the first kind of Friedmann model is that the universe is not infinite in space, but neither does space have any boundary. Gravity is so strong that space is bent round onto itself, making it rather like the surface of the Earth. If one keeps traveling in a certain direction on the surface of the Earth, one never comes up against an impassable barrier or falls over the edge, but eventually comes back to where one started.

The fourth dimension—time—is also finite in extent, but it is like a line with two ends or boundaries, a beginning and an end.

We shall see later that when one combines general relativity with the uncertainty principle of quantum mechanics, it is possible for both space and time to be finite without any edges or boundaries.

The idea that one could go right around the universe and end up where one started makes good science fiction, but it doesn't have much practical significance because it can be shown that the universe would recollapse to zero size before one could get round.

You would need to travel faster than light in order to end up where you started before the universe came to an end—and that is not allowed. But which Friedmann model describes our universe? Will the universe eventu- ally stop expanding and start contracting, or will it expand forever?

To answer this question we need to know the present rate of expansion of the universe and its present average density. If the density is less than a certain critical value, determined by the rate of expansion, the gravitational attraction will be too weak to halt the expansion.

If the density is greater than the critical value, gravity will stop the expansion at some time in the future and cause the universe to recollapse. We can determine the present rate of expansion by measuring the velocities at which other galaxies are moving away from us, using the Doppler effect.

However, the distances to the galaxies are not very well known because we can only measure them indirectly. So all we know is that the universe is expanding by between 5 percent and 10 percent every thousand million years. However, our uncertainty about the present average density of the universe is even greater.

If we add up the masses of all the stars that we can see in our galaxy and other galaxies, the total is less than one-hundredth of the amount required to halt the expansion of the universe, even in the lowest estimate of the rate of expan- sion.

But we know that our galaxy and other galaxies must contain a large amount of dark matter which we cannot see directly, but which we know must be there because of the influence of its gravitational attraction on the orbits of stars and gas in the galaxies. Moreover, most galaxies are found in clusters, and we can similarly infer the presence of yet more dark matter in between the galaxies in these clusters by its effect on the motion of the galaxies.

When we add up all this dark matter, we still get only about one-tenth of the amount required to halt the expansion. However, there might be some other form of matter which we have not yet detected and which might still raise the average density of the universe up to the critical value needed to halt the expansion. The present evidence, therefore, suggests that the universe will probably expand forever.

This should not unduly worry us since by that time, unless we have colonies beyond the solar system, mankind will long since have died out, extinguished along with the death of our sun. THE BIG BANG All of the Friedmann solutions have the feature that at some time in the past, between ten and twenty thousand million years ago, the distance between neighboring galaxies must have been zero. At that time, which we call the big bang, the density of the universe and the curvature of space-time would have been infinite.

All our theories of science are formulated on the assumption that space—time is smooth and nearly flat, so they would all break down at the big bang singu- larity, where the curvature of space—time is infinite. This means that even if there were events before the big bang, one could not use them to determine what would happen afterward, because predictability would break down at the big bang. As far as we are concerned, events before the big bang can have no consequences, so they should not form part of a scientific model of the universe.

We should therefore cut them out of the model and say that time had a beginning at the big bang. Many people do not like the idea that time has a beginning, probably because it smacks of divine intervention. The Catholic church, on the other hand, had seized on the big bang model and in officially pronounced it to be in accordance with the Bible.

There were a number of attempts to avoid the con- clusion that there had been a big bang. The proposal that gained widest support was called the steady state theory. It was suggested in by two refugees from Nazi—occupied Austria, Hermann Bondi and Thomas Gold, together with the Briton Fred Hoyle, who had worked with them on the development of radar during the war.

The idea was that as the galaxies moved away from each other, new galaxies were continually forming in the gaps in between, from new matter that was being continually created. The universe would therefore look roughly the same at all times as well as at all points of space. The steady state theory required a modification of general relativity to allow for the continual creation of matter, but the rate that was involved was so low—about one particle per cubic kilometer per year—that it was not in con- flict with experiment.

One of these predictions was that the number of galaxies or sim- ilar objects in any given volume of space should be the same wherever and whenever we look in the universe. In the late s and early s, a survey of sources of radio waves from outer space was carried out at Cambridge by a group of astronomers led by Martin Ryle. The Cambridge group showed that most of these radio sources must lie outside our galaxy, and also that there were many more weak sources than strong ones.

They interpreted the weak sources as being the more distant ones, and the stronger ones as being near. Then there appeared to be fewer sources per unit volume of space for the nearby sources than for the distant ones. This could have meant that we were at the center of a great region in the uni- verse in which the sources were fewer than elsewhere. Alternatively, it could have meant that the sources were more numerous in the past, at the time that the radio waves left on their journey to us, than they are now.

Either explana- tion contradicted the predictions of the steady state theory. Moreover, the discovery of the microwave radiation by Penzias and Wilson in also indi- cated that the universe must have been much denser in the past.

The steady state theory therefore had regretfully to be abandoned. So it is not surprising that at some time in the past they were all at the same place. In the real universe, however, the galaxies are not just moving directly away from each other—they also have small sideways velocities. So in reality they need never have been all at exactly the same place, only very close together. Perhaps, then, the current expanding universe resulted not from a big bang singularity, but from an earlier contracting phase; as the universe had col- lapsed, the particles in it might not have all collided, but they might have flown past and then away from each other, producing the present expansion of the universe.

How then could we tell whether the real universe should have started out with a big bang? But they claimed that this was still only possible in certain exceptional models in which the galaxies were all moving in just the right way. They argued that since there seemed to be infinitely more Friedmann-like models without a big bang singularity than there were with one, we should conclude that it was very unlikely that there had been a big bang.

They later realized, however, that there was a much more general class of Friedmann-like models which did have singularities, and in which the galaxies did not have to be moving in any special way. They there- fore withdrew their claim in The work of Lifshitz and Khalatnikov was valuable because it showed that the universe could have had a singularity—a big bang—if the general theory of rel- ativity was correct. However, it did not resolve the crucial question: Does gen- eral relativity predict that our universe should have the big bang, a beginning of time?

The answer to this came out of a completely different approach start- ed by a British physicist, Roger Penrose, in He used the way light cones behave in general relativity, and the fact that gravity is always attractive, to show that a star that collapses under its own gravity is trapped in a region whose boundary eventually shrinks to zero size.

This means that all the matter in the star will be compressed into a region of zero volume, so the density of matter and the curvature of space-time become infinite. In other words, one has a sin- gularity contained within a region of space-time known as a black hole. However, at the time that Penrose produced his theorem, I was a research student desperately look- ing for a problem with which to complete my Ph. So I could use it to prove that there should be a singularity only if the universe was expanding fast enough to avoid collapsing again, because only that Friedmann model was infinite in space.

During the next few years I developed new mathematical techniques to remove this and other technical conditions from the theorems that proved that singularities must occur. The final result was a joint paper by Penrose and myself in , which proved that there must have been a big bang singu- larity provided only that general relativity is correct and that the universe contains as much matter as we observe.

However, one cannot really argue with the mathematical theorem. So it is now generally accepted that the universe must have a beginning. It was coined in by the American scientist John Wheeler as a graphic description of an idea that goes back at least two hundred years.

At that time there were two theories about light. One was that it was composed of particles; the other was that it was made of waves. We now know that really both theories are correct.

Under the theory that light was made up of waves, it was not clear how it would respond to gravity. But if light were composed of parti- cles, one might expect them to be affected by gravity in the same way that cannonballs, rockets, and planets are.

In it, he point- ed out that a star that was sufficiently massive and compact would have such a strong gravitational field that light could not escape. Any light emitted from the surface of the star would be dragged back by the star's gravitational attraction before it could get very far. Michell suggested that there might be a large number of stars like this. Although we would not be able to see them because the light from them would not reach us, we would still feel their grav- itational attraction.

Such objects are what we now call black holes, because that is what they are—black voids in space. Interestingly enough, he included it in only the first and second editions of his book, The System of the World, and left it out of later editions; perhaps he decided that it was a crazy idea.

In fact, it is not really consistent to treat light like cannon- balls in Newton's theory of gravity because the speed of light is fixed. A can- nonball fired upward from the Earth will be slowed down by gravity and will eventually stop and fall back.

A photon, however, must continue upward at a constant speed. How, then, can Newtonian gravity affect light? A consistent theory of how gravity affects light did not come until Einstein proposed gen- eral relativity in ; and even then it was a long time before the implica- tions of the theory for massive stars were worked out. To understand how a black hole might be formed, we first need an understand- ing of the life cycle of a star.

A star is formed when a large amount of gas, most- ly hydrogen, starts to collapse in on itself due to its gravitational attraction. As it contracts, the atoms of the gas collide with each other more and more fre- quently and at greater and greater speeds—the gas heats up.

Eventually the gas will be so hot that when the hydrogen atoms collide they no longer bounce off each other but instead merge with each other to form helium atoms. The heat released in this reaction, which is like a controlled hydrogen bomb, is what makes the stars shine.

It is a bit like a balloon where there is a balance between the pressure of the air inside, which is trying to make the balloon expand, and the tension in the rubber, which is trying to make the balloon smaller. The stars will remain stable like this for a long time, with the heat from the nuclear reactions balancing the gravitational attraction.

Eventually, however, the star will run out of its hydrogen and other nuclear fuels. And paradoxical- ly, the more fuel a star starts off with, the sooner it runs out. This is because the more massive the star is, the hotter it needs to be to balance its gravita- tional attraction.

And the hotter it is, the faster it will use up its fuel. Our sun has probably got enough fuel for another five thousand million years or so, but more massive stars can use up their fuel in as little as one hundred million years, much less than the age of the universe. When the star runs out of fuel, it will start to cool off and so to contract. What might happen to it then was only first understood at the end of the s. Eddington was an expert on general relativity. The idea was this: When the star becomes small, the matter particles get very near each other.

But the Pauli exclusion principle says that two matter particles cannot have both the same position and the same velocity. The mat- ter particles must therefore have very different velocities. This makes them move away from each other, and so tends to make the star expand. A star can therefore maintain itself at a constant radius by a balance between the attrac- tion of gravity and the repulsion that arises from the exclusion principle, just as earlier in its life the gravity was balanced by the heat.

Chandrasekhar realized, however, that there is a limit to the repulsion that the exclusion principle can provide. The theory of relativity limits the maximum difference in the velocities of the matter particles in the star to the speed of light.

This meant that when the star got sufficiently dense, the repulsion caused by the exclusion principle would be less than the attraction of gravity. Chandrasekhar calculated that a cold star of more than about one and a half times the mass of the sun would not be able to support itself against its own gravity. This mass is now known as the Chandrasekhar limit. If a star's mass is less than the Chandrasekhar limit, it can eventually stop contracting and settle down to a possible final state as a white dwarf with a radius of a few thousand miles and a density of hundreds of tons per cubic inch.

A white dwarf is supported by the exclusion principle repulsion between the electrons in its matter. We observe a large number of these white dwarf stars. One of the first to be discovered is the star that is orbiting around Sirius, the brightest star in the night sky. It was also realized that there was another possible final state for a star also with a limiting mass of about one or two times the mass of the sun, but much smaller than even the white dwarf.

These stars would be supported by the exclusion principle repulsion between the neutrons and protons, rather than between the electrons. They were therefore called neutron stars. They would have had a radius of only ten miles or so and a density of hundreds of millions of tons per cubic inch. At the time they were first predicted, there was no way that neutron stars could have been observed, and they were not detected until much later.

Stars with masses above the Chandrasekhar limit, on the other hand, have a big problem when they come to the end of their fuel. How would it know that it had to lose weight? And even if every star managed to lose enough mass, what would happen if you added more mass to a white dwarf or neutron star to take it over the limit?

Would it collapse to infinite density? Eddington was shocked by the implications of this and refused to believe Chandrasekhar's result. He thought it was simply not possible that a star could collapse to a point. This was the view of most scientists. Einstein himself wrote a paper in which he claimed that stars would not shrink to zero size. The hos- tility of other scientists, particularly of Eddington, his former teacher and the leading authority on the structure of stars, persuaded Chandrasekhar to aban- don this line of work and turn instead to other problems in astronomy.

However, when he was awarded the Nobel Prize in , it was, at least in part, for his early work on the limiting mass of cold stars. Chandrasekhar had shown that the exclusion principle could not halt the col- lapse of a star more massive than the Chandrasekhar limit. But the problem of understanding what would happen to such a star, according to general relativ- ity, was not solved until by a young American, Robert Oppenheimer.

His result, however, suggested that there would be no observational consequences that could be detected by the telescopes of the day. And after the war the problem of gravitational collapse was largely forgotten as most scientists were then interested in what happens on the scale of the atom and its nucleus.

In the s, however, interest in the large-scale prob- lems of astronomy and cosmology was revived by a great increase in the num- ber and range of astronomical observations brought about by the application of modern technology. The light cones, which indicate the paths followed in space and time by flashes of light emit- ted from their tips, are bent slightly inward near the surface of the star. This can be seen in the bending of light from distant stars that is observed during an eclipse of the sun.

As the star contracts, the gravitational field at its surface gets stronger and the light cones get bent inward more. This makes it more difficult for light from the star to escape, and the light appears dimmer and redder to an observer at a distance. According to the theory of relativity, nothing can travel faster than light.

Thus, if light cannot escape, neither can anything else. Everything is dragged back by the gravitational field. So one has a set of events, a region of space—time, from which it is not possible to escape to reach a distant observer. This region is what we now call a black hole. If you see a Google Drive link instead of source url, means that the file witch you will get after approval is just a summary of original book or the file has been already removed.

Loved each and every part of this book. I will definitely recommend this book to science, non fiction lovers. Your Rating:. Your Comment:.